1. Field of the Invention
The present invention relates to an electrical supply unit for material to be treated in a device for the electrochemical treatment thereof, especially in galvanising or etching systems.
1. Description of the Related Art
For the electrolytic application of a conductive layer (galvanising), a material to be treated is connected via electrical feeds and fastening means to a negative pole of a direct-current source. An opposite electrode, in this case the anode, is accordingly connected electrically conductively to a positive pole of the direct-current source. Both the anode and the material to be treated lie in an electrolyte which contains positively charged ions of the material to be applied. Because of the electric field which is formed between the anode and the material to be treated, they migrate to this material to be treated and are deposited there.
The polarities are correspondingly reversed in electrolytic etching, the material to be treated then being connected to the positive pole of the current source.
At least on the end next to the material to be treated, the electrical feeds are usually designed as contact means in the form of terminal clips, tongs or clamps, so that they can hold the material to be treated. At least the interior of the entire electrical feed, including this contact means, must then consist of an electrically conductive material and be dimensioned so that the heavy currents encountered in practice can be transmitted to the material to be treated with only minor power loss and heating. The exact dimensioning of the electrical feeds depends on the electrically conductive material which is used, a larger conduction cross section being required in the case of a more poorly conducting material, for example.
The electrical feeds should then be electrically insulated on the surface as much as possible, in order to prevent large amounts of metal from also being deposited on the electrical feed during the galvanising process, that is to say when metal is being deposited on the material to be treated. In such a case, the metal needs to be removed from the electrical feed in a subsequent demetallising process, in order to avoid possible interference with the transmission of current from the contact means of the electrical feed to the material to be treated, due to these build-ups of metal. Indeed, approximately two to ten times the amount of metal is deposited on blank parts of the electrical feed compared with the surface of the material to be treated. This is because in an imaginary resistor circuit from the direct-current source to the material to be treated, the electrical feed is arranged closer to this direct-current source and therefore at a correspondingly higher electrical potential, so that field-line concentration takes place there and in turn causes greater deposition of metal.
Such a blank electrical feed also acts as a so-called robber cathode in relation to the material to be treated. Specifically, in the immediate vicinity of the electrical feed, the undesired metal deposit on the electrical feed leads to a reduction in the thickness of the metal layer on the material to be treated. For example when printed circuit boards are being coated, for which very uniform layers are important, this leads to unusable reject boards in a subsequent etching step.
In order to avoid this problem, it is known to electrically insulate all the surfaces of the electrical feed that may come in contact with the electrolyte, apart from those sections of the contact means which are used for making electrical contact with the material to be treated, for example contact areas. This is because no metal can deposit on electrically insulated surfaces during a galvanising method.
This insulation is generally provided by special plastics which have a high chemical stability, a high thermal stability and a high abrasion resistance. This special plastic insulator layer is applied by immersion or spraying with subsequent curing of the layer. If the contact means, for example a contact area, has also been insulated during such an immersion or spraying process, then it needs to be mechanically uncovered after the after curing, for example by grinding or milling. If a thicker layer is to be applied owing to more demanding requirements for the strength of the insulation, it is necessary to repeat the immersion or spraying process with subsequent curing in each case.
In order for this applied insulator layer to adhere well to the electrical feed, the electrical feed must be thoroughly cleaned before the coating and often subjected to an extra treatment, for example by grinding or sandblasting, in order to approve the adhesion.
Despite the elaborate processing and the use of high-quality special plastics such as Halar™, such insulator layers are repeatedly damaged by a sharpedged material to be treated, especially when automatic coating units are used for galvanising devices. The insulation is broken through at these points of damage, and metal will be deposited. Often after only a short time, this metal can no longer be removed by inexpensive electrolytic demetallising because, with this method, the conducting connection to the usually relatively small-area points of damage is frequently broken before all the deposited metal can be dissolved. In this case, the relevant electrical feeds need to be dismantled and demetallised chemically. If this has to be done at intervals which are too short, the electrical feed will be replaced by a new one. If an expensive metal has been used for the electrical feed, then elaborate treatment is needed in which the nonconducting plastic insulator layer is removed by melting, burning off or mechanical processing, the metal surface is re-treated and the electrical feed is re-coated with a plastic insulator layer. All these working steps are elaborate and very expensive.
An example of such an electrical feed is represented in FIG. 8. In this case, an upper electrical feed 2 and a lower electrical feed 3 respectively end in contact elements 12. For example, the lower electrical feed 3 is fastened to a chain circulating in an electrolyte bath, or a toothed belt, and is moved into or out of the plane of the drawing because of a corresponding drive. The upper electrical feed 2 is mounted so that it can move in a vertical direction, as indicated by the arrow, and is pressed downwards by a compression spring. The electrical feeds 2 and 3 thus form a clamp, the upper electrical feed 2 constituting a clamp upper part and the lower electrical feed 3 constituting a clamp lower part. A material 1 to be treated can therefore be clamped between the two contacts 12.
Using a thrust block (not shown) on the upper electrical feed 2, and by means of a sloping plane on an upper feed of the electrical feed, the clamp formed in this way can be closed when entering a galvanising region and re-opened when leaving this region. When it is closed, the clamp therefore engages with the material 1 to be treated and makes the electrical connection to the corresponding pole of the electrical supply. When leaving the galvanising region, the electrical feed is re-opened and the material 1 to be treated is transported further by means of a roller path.
In order to prevent the metallic upper and lower electrical feeds 2, 3 from being galvanised and acting as robber cathodes, they are provided with the aforementioned thin plastic insulator layer as described above, at least to above the liquid level 21 of the galvanising bath. This insulation layer extends as far as the side face of the contacts 12.
An alternative solution approach is known from DE 197 35 352 C1. In this case, electrical feeds made of a blank conductive material are used without a plastic insulator layer. In order to avoid excessive deposition of metal on the electrical feed, masks are provided which are arranged extensively along an entire galvanising cell in which the electrical feed is used. The masks are in this case fastened rigidly to a housing of the galvanising cell, or to the anodes. The masks are made of an electrically insulating material, at least on their surface, and they are arranged so that only minor migration of the ions contained in the electrolyte, especially the metal ions, can take place to the electrical feeds when galvanising, since these ions cannot penetrate the electrically insulating masks and no current can flow through the latter. The ions take the path of least resistance instead, in this case the intended path to the material to be treated. In this context, screening of the field lines in the electrolyte by the masks is also referred to.
A gap in the masks must in this case be arranged so that a maximum thickness of the material to be treated, for which the device is designed, passes through these gaps but without touching the masks.
If thin material to be treated, that is to say with a thickness less than the maximum thickness, is being treated in the system, then this leads to a larger clearance between the material to be treated and the masks. So great an amount of ions can then migrate to the contact elements that, in spite of the masks, relatively thick metal layers with a thickness of about 0.1 mm or more per pass can be deposited on the electrical feeds in the vicinity of the gaps. These metal layers can then no longer be removed by subsequent electrolytic demetallising, as mentioned above, in the time available before the next pass.
Easy removal of such an undesired metal layer on or in the contact elements during operation is readily possible in horizontal galvanising systems for the copper-plating of printed circuit boards with a thickness of up to about 0.04 mm. Beyond a layer thickness of about 0.05 mm to 0.1 mm for the undesired metal layer, depending on the system configuration, the layer can no longer be removed reliably from all points of the contact elements. The layers accumulate during the subsequent passes and must be elaborately removed in operational pauses. With a layer thickness beyond 0.1 mm per pass for this layer, interference with production is to be expected.
Another disadvantage with this embodiment is that, in the case of electrical feeds arranged as in FIG. 8, the masks cannot be moved together with the engaging movements of the electrical feeds.
WO99/29931 discloses a clamp-like holding device for releasably holding objects, such as printed circuit boards, to be galvanised by means of dip galvanising. The holding device comprises a first bar, a second bar and mutually opposing contact pins for making contact with and clamping the printed circuit board at lower end regions of the first and second bars. A sleeve which is elastically deformable in the axial direction of the contact pins, for example in the form of a bellows, is fastened to each of the contact pins and, in the relaxed state, extends beyond the contact area of the contact pin. When the clamp is holding a printed circuit board, the free ends of the sleeves lie tightly on the surface of the printed circuit board and therefore prevent the contact areas from coming in contact with the galvanitic bath, so as to prevent metallic deposits on the contact areas.
DE 42 11 253 A1 discloses a galvanising unit in which the work-pieces to be galvanised are transported in a horizontal pass through an electrolyte. The cathodic contact with the work-pieces travelling through is in this case made by rotatable contacting wheels, with covers of insulating material being applied to end sides of the contacting wheels in order to prevent undesired metal deposits on the end sides.
DE 100 43 815 C2 also discloses a galvanising unit. In this case, contact is made with the material to be treated by strip-shaped contact elements. For example, a partially vulcanised electrically insulating material is used in order to prevent undesired metallisation of the contact strips with the contact elements.
DE 100 65 643 C2 also uses contact strips for making contact with material to be treated, the contact strips comprising contact insulations which fully cover the contact strips apart from an actual contact area.
WO 03/071009 A1 discloses another galvanising unit, in which an electrical contact strip is built into an electrically insulating shaft, with the insulating shaft protecting the contact strips against undesired metallisation.